1. Field of the Invention
The present invention relates generally to the pneumatic control of fluid process control valves and, specifically, to an improved mechanically actuated pilot valve for such pneumatic control systems.
2. Description of the Prior Art
The oil and gas, chemical and other industries routinely use pneumatically operated valves to regulate process levels and pressures in both lines and vessels. The controller for these valves generally contains a sensing element such as a Bourdon tube or a float mechanism. The sensing element responds to the process pressure or fluid level, providing mechanical movement to a "flapper" or other mechanical actuator element when a process change occurs.
In the case of controlling fluid level in a vessel, there exists in the control line of the discharge valve an open gas pilot which bleeds to the atmosphere. While the gas pilot is unobstructed the control line is at low pressure and the discharge valve is closed. There exists within the vessel a float. When the fluid level in the vessel rises the float also rises which causes a flapper element outside of the vessel to move. The flapper element moves to block the escape of control gas from the open pilot to the atmosphere, causing a rise in gas pressure in the control line. This rise in control gas pressure causes the pneumatic, diaphragm operated discharge valve to open, allowing fluid to escape from the vessel. In this manner, the level of fluid in the vessel is controlled.
The constant escape of the compressed control gas to the atmosphere from the open gas pilot is expensive and in the case of natural gas, poses significant environmental and safety risks. Therefore a need exists for a "non-bleed" pneumatic control pilot valve which reduces and/or restricts the flow of control gas to the atmosphere during the control operation. Such non-bleed pilot valves to date have relied on differential sealing surfaces and diaphragms or other dynamic seals to provide gas output based on force applied by the flapper element. The force necessary to operate these force-sensitive pilots causes the flapper element to become a "sprung" element in the control system. Additionally, the essentially unrestricted flow of gas through these pilots loads the internal members and gives unpredictable control response times. The resulting loading and unloading of the flapper element during operation of these pilots introduces undesirable and often unusable control responses to the process control valve.
The present invention has as its object to provide an improved, "non-bleed" pilot valve which responds to flapper position instead of applied flapper force.
Another object of the invention is to provide such an improved pilot valve with a mechanism which reduces the necessary actuating energy, thereby reducing the tendency of the flapper element to become "sprung" in the system being controlled.
Another object of the invention is to provide an improved pilot valve, the operation of which provides a normally restricted flow of gas through the valve, thereby resulting in smooth and predictable control responses.